ZTL CTLR EXPERIENCED OPERROR AT 17000 WHEN FAILING TO TAKE TIMELY ACTION BETWEEN A CLIMBING AND DESCENDING ACFT.

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ACR X WAS AN INBOUND TO CLT ON THE SUDSY 1 ARR 220 DOWN TO 130. ACFT Y WAS A BE40 DEP OFF OF SVH NEBOUND TO BDL 115 UP TO 230. ACFT Y WAS ORIGINALLY ON A HDG TO THE NE WHERE IF I HAD LEFT HIM ALONE WOULD HAVE SEPARATED HIM FROM THE ACR X. I VECTORED HIM TO THE R ATTEMPTING TO CROSS HIM OUT WITH ACR X WHO WAS DSNDING ON THE ARR. THIS WOULD HAVE ENABLED ACFT Y TO BE ON COURSE SOONER. I HAD ORIGINALLY THOUGHT THE ACFT Y WOULD CLB VERY WELL. THERE WAS AT LEAST 50-60 KTS ON HIS TAIL. I REALIZED THIS AND SHORTLY AFTER I GAVE HIM THE CLRNC I WAS SKEPTICAL THAT THIS WOULD WORK. HOWEVER INSTEAD OF ENDING IT AT THAT TIME AND STEPPING BOTH ACFT OFF AT ALT; I DECIDED TO GIVE IT A LITTLE MORE TIME. I EVEN ASKED BOTH ACFT TO KEEP A GOOD RATE OF DSCNT AND A GOOD RATE OF CLB. WHEN I FINALLY DECIDED IT WAS NOT GOING TO WORK I ATTEMPTED TO STEP BOTH ACFT OFF. THE FIRST TIME I TRIED TO STEP ACFT Y OFF IT WAS ANSWERED BY ANOTHER ACFT ON FREQ. ON THE SECOND ATTEMPT ACFT Y ACKNOWLEDGED THE CHANGE OF ALT. I ATTEMPTED TO STEP ACFT X OFF AT 170 3 TIMES. ON THE 3RD TIME HE ACKNOWLEDGED. I ASKED ACFT Y TO VERIFY HIS ALT AT 160. HE RESPONDED IN THE NEGATIVE AND THAT HE HAD THE ACFT X IN SIGHT. THE ACR X RPTED THAT HE WAS AT 170 AND HAD TURNED FOR THE ACFT Y. I CANNOT EXPLAIN TO MYSELF LET ALONE SOMEONE ELSE HOW I COULD LET A SIMPLE SITUATION GET OUT OF MY CTL.